By synthesizing sound signals generated from two microphone units by using a variable directional microphone configured by the two microphone units, directivity such as omnidirectivity, cardioid, hypercardioid, or bidirectivity can be obtained selectively.
In this case, as both of the two microphone units, unidirectional microphone units are used, and the microphone units are arranged coaxially so that the directivity axes thereof are directed to the directions opposite to each other (180° directions) (for example, refer to Patent Document 1 (Japanese Patent Application Publication No. 2005-184347)).
Therefore, as the microphone unit, a small-size unidirectional condenser microphone has been used frequently, and a dynamic microphone unit has scarcely been used because of its large size.
The reason why the dynamic microphone unit is large in size is that a rear air chamber is needed to obtain an omnidirectional component regardless of whether it is omnidirectional or unidirectional. FIG. 10A is a sectional view showing the schematic configuration of the dynamic microphone unit, and FIG. 10B is an equivalent circuit diagram of the dynamic microphone unit shown in FIG. 10A.
As shown in FIG. 10A, a dynamic microphone unit 1 includes, as a basic configuration, an electrokinetic acousto-electric converter 1a and a rear air chamber 1b. 
The electrokinetic acousto-electric converter 1a has a diaphragm 10 having a voice coil 11 and a magnetic circuit section 20 having a magnetic gap 21 in which the voice coil 11 are oscillatably arranged. The magnetic circuit section 20 is housed in a cylindrical unit holder 30. The diaphragm 10 is supported on a peripheral edge portion of an enlarged-diameter flange part 31 of a unit holder 20.
Since this dynamic microphone unit 1 is unidirectional, the flange part 31 is provided with a bidirectional component intake port (rear acoustic terminal) 32 communicating with a front air chamber 12 existing on the back surface side of the diaphragm 10. In the case where the dynamic microphone unit 1 is omnidirectional, the bidirectional component intake port 32 is not provided.
The rear air chamber 1b is formed by a substantially enclosed unit case 40 mounted on the rear end side of the unit holder 30. The front air chamber 12 on the diaphragm 10 side and the rear air chamber 1b are connected acoustically to each other via a sound wave passage in the unit holder 30. In the sound wave passage, a predetermined acoustic resistance material 33 is provided.
In the equivalent circuit diagram of FIG. 10B, P denotes a front sound source, Pe−jkd cos θ denotes a rear sound source, m0 and S0 denote the mass and stiffness of the diaphragm 10, respectively, S1 denotes the stiffness of the front air chamber 12, r0 and m1 denote the resistance and mass of the bidirectional component intake port 32, respectively, r1 denotes the braking resistance of the acoustic resistance material 33, and S2 denotes the stiffness of the rear air chamber 1b. 
The low frequency limit in the frequency characteristics is mainly determined by the mass and compliance (1/S0) of the diaphragm 10. However, in the case where the capacity Ca of the rear air chamber 1b is low, the low frequency limit is affected. Therefore, in the dynamic microphone unit 1, the capacity Ca of the rear air chamber 1b must be increased. Accordingly, the external dimensions of the dynamic microphone unit 1 become far larger than those of the condenser microphone unit. The large capacity Ca of the rear air chamber 1b exerts an influence on a low frequency (omnidirectional component) only, and scarcely exerts an influence on the frequency band (bidirectional component) in which the unidirectivity is obtained.
In the case where the variable directional microphone is configured by a pair of above-described dynamic microphone units 1, a series mode in which the two dynamic microphone units 1 are arranged coaxially in a back-to-back form as shown in FIG. 11A and a parallel mode in which the rear air chambers 1b of the two dynamic microphone units 1 are lapped on each other as shown in FIG. 11B are conceivable.
In the series mode shown in FIG. 11A, unfortunately, the overall length becomes double the length of the dynamic microphone unit 1, and accordingly the distance between the acoustic terminals of the dynamic microphone units 1 also increases. Therefore, there arises a problem that the difference (phase difference) between arrival times of sound waves to the acoustic terminals from the sound source increases, so that turbulence is easily produced especially in a high sound range.
In contrast, according to the parallel mode shown in FIG. 11B, the overall length can be shortened as compared with the series mode. However, the acoustic terminals of the dynamic microphone units 1 are arranged asymmetrically in the right-and-left direction. Therefore, there arises a problem of deteriorated directional frequency response.
Accordingly, an object of the present invention is to provide a variable directional microphone including dynamic microphone units that is small in size and has good directional frequency response.